Field of the Invention
The invention relates to a method for channel assignment, and to a radio communications system, in particular a mobile radio system with time division duplex (TDD) subscriber separation.
In radio communications systems, information (for example voice, video information or other data) is transmitted with the aid of electromagnetic waves via a radio interface between a transmitting radio station and a receiving radio station (base station and mobile station, respectively). The electromagnetic waves are in this case transmitted at carrier frequencies that are in the frequency band intended for the respective system. Frequencies in the frequency band at about 2000 MHz are intended to be used for future mobile radio systems using CDMA or TD/CDMA transmission methods via the radio interface, for example the universal mobile telecommunication system (UMTS) or other third-generation systems.
Frequency division multiple access (FDMA), time division multiple access (TDMA) or a method known as code division multiple access (CDMA) are used to distinguish between signal sources, and thus for evaluation of the signals. One particular version of time division multiple access is a TDD (time division duplex) transmission method, in which the transmission is made in a common frequency band, separated in time both in the up link direction, that is to say from the mobile station to the base station, and in the down link direction, that is to say from the base station to the mobile station.
The TDD transmission method supports, in particular, asymmetric services in which the transmission capacity in the up link direction does not need to be the same as that in the down link direction. As a rule, the asymmetry will not be desirable to the same extent in all the radio cells, thus resulting in a critical interference scenario. The analyses relating to the TDD transmission method that have been carried out so far have underestimated the problems of mutual interference between a large number of base stations and mobile stations which are being operated in the same frequency band.
It is accordingly an object of the invention to provide a method for channel assignment and a radio communications system that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which interference is further reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for assigning channels in a radio communications system, which includes:
using a time division duplex transmission method with timeslots as channels for a radio transmission between a base station and mobile stations and with switching points separating the timeslots for an up-link direction and the timeslots for a down-link direction;
defining in each case at least one transmission parameter relating to a transmission quality for the radio transmission between the base station and the mobile stations; and assigning the timeslots close to a switching point of the switching points to the mobile stations having a qualitatively better transmission parameter.
In the method for channel assignment according to the invention, a TDD transmission method with timeslots as channels is used for radio transmission between the base station and the mobile stations, with a switching point separating timeslots for the up link direction and timeslots for the down link direction. A transmission parameter relating to the transmission quality is in each case defined for the radio transmission between the base station and the plurality of mobile stations. The mobile stations with the qualitatively better transmission parameter are preferably assigned timeslots close to the switching point.
If the switching points for adjacent radio cells differ, then interference is unavoidable despite frame synchronization. However, since the links to mobile stations with good transmission quality are disposed around the switching point, then their transmission power, inter alia, can be set on the basis of the good transmission conditions to a minimal interference influence for the other mobile stations and base stations. This reduces the interference between radio cells in the radio communications system.
According to an advantageous development of the invention, an area switching point is defined in a group of adjacent base stations, and the mobile stations with the qualitatively better transmission parameter are preferably assigned timeslots between the radio-cell-specific switching point and the area switching point. This reduces the interference not only with respect to a cell, but in at least one subregion of the radio communications system. The area switching point indicates an average value for the asymmetry, on an area basis. A first upper boundary is advantageously defined for the difference between the radio-cell-specific switching point and the area switching point. If there is little interference between adjacent cells, for example between different rooms when indoors, then a high upper boundary can be set. It is likewise advantageous for the first upper boundary to be set as a function of the number of mobile stations for which a qualitatively good transmission parameter has been determined. The boundary thus does not place any excessive limitation on the transmission capacity if the timeslots around the switching point can scarcely be filled when there are few mobile stations with good transmission parameters.
It is within the context of the invention for a base switching point to be defined in the radio communications system, from which base switching point the radio-cell-specific switching points and the area switching points may have an error of only a maximum of a second upper boundary. This measure assists the administration of the switching points throughout the network, and keeps interference beyond area boundaries low.
In this case, it is advantageous for buffer zones, with radio cells in which the base switching point is binding, to be administered between groups of base stations with different area switching points. This provides a low-interference transition between areas that have a very different requirement with regard to the asymmetry of radio transmission.
The introduction of area switching points and a base switching point results in a frame within which the individual base stations can set the radio-cell-specific switching point as required, without this having to be harmonized by using higher function layers.
Particularly for transmission methods which use a number of switching points within a frame, an alternative embodiment of the invention defines a standard TDD scheme to which, however, the base stations in the individual radio cells are not bound. In order to reduce interference, the mobile stations with the qualitatively better transmission parameters are preferably assigned timeslots which are used, rather than the standard TDD scheme, for the opposite transmission direction. A change from the standard TDD scheme leads to one or more timeslots no longer being used for the up link direction, but for the down link direction (or vice versa). Since it can be assumed from this that the standard TDD scheme is being used in the adjacent radio cell, these timeslots which are susceptible to interference are assigned to the links to mobile stations with good transmission quality.
It is particularly advantageous for the change from the standard TDD scheme to be made in a pre-set manner or in a manner that has been negotiated with the adjacent base stations, that is to say, of a number of possible timeslots which can be used in the opposite transmission direction, a number of base stations as far as possible use the same timeslots for this purpose. This results in less interference, despite a change from the standard TDD scheme.
The transmission parameter or parameters which forms or form the basis for channel assignment is or are related to a signal delay time between the base station and the mobile station, to a reception power level in the respective receiving station, to the influence of interference on the radio transmission from and/or to the mobile station on adjacent radio cells, and/or on the position of the mobile station within the radio cells. These variables can be derived, without any major additional effort, from the transmission characteristics that are normally determined for the radio link between the base station and the mobile station. Corresponding to the good transmission quality, the transmission relationships for the mobile stations with timeslots around the switching point are set to achieve low interference levels for the adjacent cells.
According to an advantageous development of the invention, radio transmission within at least parts of the radio communications system is set to be frame-synchronous, and the switching points of adjacent base stations are set as a function of the traffic. Frame synchronization reduces the interference. Setting the switching point as a function of traffic leads to improved utilization of the transmission capacity, since the up link direction or the down link direction may be preferred, depending on the requirement.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for channel assignment and a radio communications system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.